PROJECT SUMMARY Motor vehicle collisions (MVCs) remain the leading cause of unintentional injury deaths among children in the United States (U.S.) and racial/ethnic minority children are disproportionately impacted as suboptimal child passenger safety behaviors are more prevalent in some communities. Existing universal approaches to promote child passenger safety have fallen short of ensuring that all child passengers are correctly using size- appropriate child passenger restraints according to guidelines published by the American Academy of Pediatrics and the National Highway Traffic Safety Administration. Precision prevention programs are urgently needed to improve child passenger safety behaviors among caregivers who have not been responsive to guidelines, laws, and public education campaigns. The proposed research will test the efficacy of Tiny Cargo, Big Deal (TCBD), an emergency department (ED)-based precision prevention intervention grounded in Self- Determination Theory. TCBD integrates personalized counseling based on principles of motivational interviewing (MI) and eHealth components including a tailored educational web application (app) and short message service (SMS) communications with the goal of improving child passenger safety. We hypothesize that by providing tailored child passenger safety education and personalized skills for restraint use in a manner that supports autonomous motivation the TCBD intervention will be more efficacious than universal approaches (laws/guidelines) for realizing correct use of size-appropriate child passenger restraints. The primary outcomes, size-appropriate restraint use and the Child Passenger Safety Score (CPaSS), will be assessed with direct observation at scheduled, in-person appointments and remote observation of digital photographs submitted by caregivers in response to unscheduled text messages. We will test our hypothesis using a randomized adaptive trial design with the following aims: Aim 1: To refine and verify the reliability of elements in the 100-point CPaSS, a novel composite measure of child passenger safety. Aim 2: To test the efficacy of the TCBD intervention compared to usual care for improving primary outcomes [i.e., size- appropriate restraint (dichotomous) and CPaSS-D (numeric)] at 6 and 12 months. Aim 3: To use latent growth curve models to identify characteristic trajectories for the CPaSS-R observed in digital photographs over the 12 study months and assess for correlates of the trajectory types. This innovative, theoretically grounded research will have long-standing impact on child passenger safety by informing the best methods to increase correct use of size-appropriate child restraint systems among caregivers of diverse backgrounds at the point they interact with the health care system. The work has the potential for wide-scale implementation in the ED and for translation to primary care offices, schools, and community settings with an ultimate goal of eliminating child passenger safety disparities and reducing the number of childhood injuries from MVCs.